Production of polyamide from lactam as a starting material is generally effected by means of melt polymerization of lactam using water as a catalyst, a combination of such melt polymerization with solid-state polymerization, anionic polymerization and the like. However, when a polyamide material obtained by such polymerization methods is used in the conventional molding and processing of fibers, films and the like, the processed products show inferior strength and modulus, e.g., merely about, 5 to 10% of the theoretical strength and modulus. Low molecular weight of such a type of polyamide can be regarded as a primary factor giving rise to these inferior properties.
For example, in the case of synthesis of nylon 6 by polymerization of .epsilon.-caprolactam, nylon 6 obtained by melt polymerization of .epsilon.-caprolactam using water as a catalyst has an inherent limitation in terms of its weight average molecular weight; a maximum weight average molecular weight of only about 100,000 can be obtained. Also, when the nylon 6 obtained by such melt polymerization followed by a solid-state polymerization, its weight average molecular weight can be increased, but with a concomitant broadened molecular weight distribution, sometimes also resulting in multimodal distribution.
In the case of anionic polymerization in which alkali metals such as sodium, potassium and the like and Grignard compounds are used as the main catalysts, nylon 6 obtained by such a process has a higher molecular weight than in the case of melt polymerization or the combination of melt polymerization with solid-state polymerization. For example, JP-B-43-1621 (the term "JP-B" as used herein means an "examined Japanese patent publication") discloses a process for anionic polymerization of .epsilon.-caprolactam in which an alkali metal and N-aluminum-.epsilon.-caprolactam are used as catalysts. However, the solvent-soluble portion of nylon 6 obtained by this process has a weight average molecular weight of only about 250,000 which is not a satisfactory level when the product is used in subsequent applications as a high strength and high modulus starting material.
JP-B-1-19807 discloses that nylon 6 having a weight average molecular weight of from 3.times.10.sup.5 to 10.times.10.sup.5 can be obtained by anionic polymerization of .epsilon.-caprolactam using a Grignard compound, an acyl lactam compound and an organo aluminum compound. However, the organo aluminum compound used in this process is neither economical nor practical to use, because not only this compound is expensive, but also it has such an unstable nature that it is decomposed easily in air which leads to a possible danger of igniting and causing combustion.
JP-B-42-26874 (corresponding to U.S. Pat. No. 3,359,227) discloses that a molded polyamide product having a markedly high strength can be obtained by the use of a tertiary amide, a dialkyl sulfoxide and a dialkyl sulfone in an amount of from 1 to 20% by weight, preferably from 3 to 7% by weight, in addition to the use of a known alkaline main catalyst and a cocatalyst. This disclosure, however, does not include data regarding viscosity and molecular weight of the molded polyamide product. In addition, metallic sodium and sodium lactamate are used as the main catalysts in the Examples of this disclosure. Such main catalysts, however, are difficult to handle because of their unstable nature in comparison with Grignard compounds, in addition to another disadvantage in that a polymerization reaction using these main catalysts frequently causes side reactions which leads to decreased polymer yield.